How the cold, dead moon stayed magnetic

A mystery thrown up by the Apollo moon rocks may finally have been solved. How did the moon remain magnetic tens of millions of years after its molten core stopped sloshing?

The moon has no global magnetic field today, but early in its life, it probably had a core hot enough to churn violently, with the movement of this electrically charged fluid creating a magnetic field. But as the core cooled, the convection should have eased enough to kill the field. So it was a puzzle when Apollo moon rocks suggested the moon still had a magnetic field 4.2 billion years ago, millions of years after the powerful mixing is thought to have ended. Now two groups have come up with explanations for what could have kept the core stirred up.

The moon is thought to have formed closer to the Earth than it is now and spun faster, slowing down and moving away over time through tidal interactions with Earth. Christina Dwyer at the University of California, Santa Cruz, and colleagues say previous models did not take into account this faster spin, which would have agitated the molten core like water in a washing machine. This could have enabled the magnetic field to last until 2.7 billion years ago.

Michael Le Bars at the Non-Equilibrium Phenomena Research Institute in Marseille, France, says large meteorite impacts that occurred until about 3.9 billion years ago also could have set the lunar core sloshing for periods of 10,000 years at a time.

Both models offer "a way out of a pretty major conundrum", says Ben Weiss at the Massachusetts Institute of Technology. Some meteorites – thought to be chips off of asteroids – are magnetic, and spacecraft flybys have measured two asteroids with magnetic fields, he says, adding that the models might explain how these space rocks came to be magnetised.

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